1. Field of the Invention
This invention relates to a magnetic disk reproducing apparatus and to a control method for controlling the magnetic disk reproducing apparatus. More particularly, this invention relates to a magnetic disk reproducing apparatus for detecting and correcting thermal asperity and to a control method for such a magnetic disk reproducing apparatus.
Recently, magnetic disk reproducing apparatuses using a magnetoresistance effect type head (MR head), which in turn uses a magnetoresistive element (MR element), have become available. Even more recently, the flying height of the MR head has been reduced to obtain a higher recording density, and a problem concerning thermal asperity, or in other words, the fluctuation of the waveform of a reproduced signal resulting from the heat of friction caused by a collision between a protrusion on a disk medium and the MR head, has appeared. Therefore, a magnetic disk reproducing apparatus (e.g., a magnetic disk drive) which is capable of correcting a reproduced signal and reading out correct data even when thermal asperity occurs, has been required.
2. Description of the Related Art
The storage capacity (i.e., memory capacity) of magnetic disk drives has been increasing in recent years. An increase in recording density is the main contributor to the increase in the storage capacity. A method of increasing the recording density can be generally classified into the following two methods. One is the method of increasing the number of data tracks in a radial direction, and the other is the method of increasing the storage capacity in a circumferential direction.
Some of the latest magnetic disk drives increase the memory capacity in the circumferential direction by using the MR element for the reproducing head and, in order to further increase the storage capacity, it has become necessary to reduce the flying height of the reproducing head (MR head) from the medium surface and to improve the S/N (signal to noise) ratio in the output of the reproducing head.
Here, the MR element has a feature that its electric resistance changes in accordance with the change of an external magnetic field. The MR head utilizes this feature of the MR element, causes a predetermined current to flow through the MR element, and picks up magnetization on the medium as a voltage signal. Unlike an inductive head, the MR head can easily extract the signal even when the medium is rotating at a low rotating speed, and is therefore an effective means for increasing the storage capacity of the magnetic disk reproducing apparatus and for reducing its size.
FIG. 1 is a schematic view useful for explaining thermal asperity that occurs when the magnetoresistance effect type head (MR head) is used. In the drawing, reference numeral 1 denotes a disk (medium); reference numeral 10 the surface of the disk medium; reference numeral 10a a protrusion; reference numeral 20 a head suspending portion; and 24 the MR head.
When the surface 10 of the disk medium 1 is observed microscopically, protrusions 10a occur on the surface 10 of the disk medium 1 due to the change in quality with the lapse of time or the like, for example, in the magnetic disk reproducing apparatus (hard disk apparatus), as shown in FIG. 1. When the flying height of the MR head is reduced so as to increase the storage capacity, the MR head impinges against the protrusions 10a on the surface of the disk medium, and thermal asperity, which brings about the fluctuation of the waveform of the reproduced signal, causes a problem.
In other words, when the flying height of the MR head is reduced to accomplish a higher recording density, a collision of the MR head against protrusions (protruding portions) occurs. This collision generates heat of friction in the MR head and brings about a change (increase) in the electric resistance due to a temperature rise in the MR device, so that thermal asperity occurs which causes a fluctuation of the waveform of the reproduced signal.
FIG. 2 is a signal waveform diagram showing the fluctuation of the DC (direct current) level of the reproduced signal due to thermal asperity.
Assuming that the protrusion 10a exists on the surface 10 of the disk medium 1 due to some problems caused during fabrication process for the disk medium, for example, as shown in FIG. 2, the MR head 24 impinges against the protrusion 10a on the disk medium, and the DC level of the reproduced signal greatly changes due to thermal asperity and in some cases, an abnormal waveform occurs.
Though this abnormal waveform returns to the original waveform within several micro seconds (.mu.sec), the data cannot be correctly demodulated during this period of several micro seconds). The portions from which the data cannot be correctly read out can be registered as a medium defect and the use of the portions can be inhibited before shipment of the apparatus. However, since the defective portion (protrusion) always repeats collisions with the head, there is a possibility that the defect on the medium surface expands in the radial/circumferential directions by the change in quality with the lapse of time, and the possibility of the occurrence of a new protrusion on the surface of the medium disk 10 due to the change in quality with the lapse of time, also exists. It is not possible to register the medium defect before shipment of the apparatus, with regard to the change (occurrence and expansion of the protrusion) of the surface condition of the disk medium due to such a change in quality with the lapse of time.
FIGS. 3 to 6 are signal waveform diagrams showing the first to fourth examples of the level fluctuation of reproduced signals by a simulation technique when thermal asperity occurs. FIGS. 3 to 6 represent the cases in which the time constants of thermal asperity are set as .tau.=200, .tau.=400, .tau.=500 and .tau.=600, respectively. Data is obtained by connecting random data at sample points with each other. A signal level (amplitude) of 1.5 is assumed as the dynamic range of the circuit. When the amplitude exceeds this value 1.5, the waveform gets into saturation and in the interim, the data cannot be demodulated.
As shown in FIGS. 3 to 6, when the MR head impinges against the protrusion on the disk medium, for example, the waveform of the reproduced signal fluctuates due to thermal asperity. In other words, because the MR head converts the resistance of the MR element to a voltage, the resistance of the MR element changes (becomes large) due to the heat of friction if the protrusion on the disk medium impinges against the MR head and generates the heat of friction, and the waveform drastically fluctuates (rises). As a result, the reproduced signal exceeds a capacity of an automatic gain control circuit (AGC circuit) which adjusts the waveform envelope to a constant value, or exceeds the dynamic range of the signal processing circuit, and the reproduced signal gets into saturation. In consequence, the recorded data cannot be demodulated. More concretely, 256 bits cannot be demodulated in FIG. 3, 521 bits in FIG. 4, 775 bits in FIG. 5 and 1,128 bits, in FIG. 6.
As described above, when thermal asperity occurs, the level of the reproduced signal drastically rises and the demodulation cannot be carried out. As the heat is radiated, the MR head whose temperature rises due to the heat of friction thereafter returns to the normal temperature, and the level fluctuation of the reproduced signal damps exponentially. Here, as obvious from FIGS. 3 to 6, the fluctuation of the reproduced level due to thermal asperity exhibits characteristics in which as the value of the time constant .tau. of thermal asperity, becomes larger the number of bits that cannot be demodulated also becomes larger. With regard to the damping of the level fluctuation, too, the level returns to the original level in accordance with a function corresponding to the value of the time constant .tau. of thermal asperity.
To address this thermal asperity, U.S. Pat. No. 5,233,482 discloses a certain solution. This prior art reference describes a technique for holding an AGC circuit (automatic gain control circuit) for adjusting the envelope of the reproduced waveform, and a technique for shortening the time necessary for holding the AGC circuit. More concretely, the above reference describes a method of eliminating the number of bits of the waveform fluctuation by changing the cutoff frequency by using an AC (alternating current) coupling capacitor, and a method of eliminating saturation of the waveform due to thermal asperity by expanding the operating range of the ADC. However, because this method is not a method for correcting thermal asperity itself, there is a limit with respect to a decrease in a time period represented by the number of bits (bit number) in which an error concerning demodulated data occurs.